The U.S. reserve of coal is about 3 trillion tons. Although the most abundant (80%) fossil fuel in America is coal, the U.S. consumption pattern is quite a reversal of form in terms of utilization, with coal representing only 17%, oil and gas about 78%.
The demand for all fossil fuels combined is expected to double by the year 2000, even with increasing the use of nuclear power. While the domestic supply of crude oil and natural gas is not likely to keep pace with the energy demand, coal can play an important role in filling such a gap and thus reduce the requirements for imported supplied of oil and gas.
Coal, the fossilized plant life of prehistoric times, contains various amounts of sulfur due to the nature of its origin. Under most existing commercial technology, the generation of electricity from coal poses environmental problems because of sulfur oxides and particulate emissions. Since most of the coals in this country, particularly the Eastern and Midwestern coals, have high sulfur content (&gt;2%) there is a need for an economical process of converting high sulfur coals to clean fuel (for example, 1.2 lbs of SO.sub.2 emission per million Btu by one EPA standard) in order to utilize coal as a source of energy without causing serious air pollution. So the need for converting massive coal reserves to clean-burning solid fuel, liquid fuel and pipeline quality gas is self evident. If the vast coal reserve is converted to clean fuel, it can supply most of the energy needs of the United States for the next three centuries.
Processes for chemically cleaning and/or liquefying coal are presently being developed. Generally in these processes, a reactive chemical agent or agents are introduced into the coal structure to act on the coal. It is therefore critical that the reactive agent quickly and homogeneously penetrate the entire coal structure to ensure complete interaction between the coal and the reactive agent and to decrease the bonding strengths of the sulfur moieties in the coal structure.
Desulfurization of coal by low temperature chlorinolysis is a particular area of chemical coal treatment where the mass transport and bond strength problems have arisen. Sulfur in coal occurs in two types, generally in approximately equal amounts of inorganic sulfur (primarily as pyrites) and organic sulfur in the forms of thiophenes, sulfides, disulfides and mercaptans chemically bound in the organic structure of the coal. Minor amounts of sulfates are also present. A typical low temperature chlorinolysis process is described in U.S. Pat. No. 4,081,250. This three-stage process includes an initial room temperature chlorine treatment of coal suspended as a slurry in a liquid phase of methylchloroform. After chlorinolysis, a batch hydrolysis and solvent recovery are carried out. Finally, dechlorination at 300 degrees C. to 450 degrees C. yields a desulfurized coal product.
This chlorinolysis process works well for removing inorganic sulfur from coal. However, removal of the more tightly bound organic sulfur has been less than adequate. Problems in removing organic sulfur are believed to be due to the problems of both mass transfer of chlorine and bond strengths of sulfur entities in the coal.
Water has also been used as a mass transfer media in low temperature chlorinolysis (PCT Patent application No. PCT/US79/00886). However, like the mixture of water and methyl chloroform, there have been problems with adequate organic sulfur removal for coals such as PSOC 190 and 276.